1. Field of the Art
This invention relates to a process for producing polyarylene thioethers having high stability. More particularly, this invention relates to a process for producing stable polyarylene thioethers by the reaction of an alkali metal sulfide and a dihaloaromatic compound in a polar organic solvent, characterized in that total charge of the haloaromatic compound (dihaloaromatic comound alone or mixture) is used in an appropriately excessive amount to the effective charge of the alkali metal sulfide and the reaction is continued until the concentration of a thiolate group or a thiol group at the resulting polyarylene thioether terminal reaches 10 .mu.mole/g or less by intense stirring for substantial mixing at least on the latter step of the reaction.
2. Prior Art
Polyarylene thioethers are thermoplastic resins having high crystallizability and can be processed into not only molded articles by injection molding or the like but also film, sheet, plate, fiber or other formed articles by extrusion molding, compression molding, inflation, melt spinning, stretching or the like. Therefore, it has been expected that formed articles by melt process having excellent properties might be obtained.
However, conventional commercially available polyarylene thioethers are mostly the resins wherein relatively low melt viscosity materials (i.e. materials of low molecular weight) is "cured" or treated at an elevated temperature in the presence of oxygen or the like to cause crosslinking, branching or the like so that the apparent melt viscosity is increased. Such resins of which melt viscosity is increased by "curing" have many problems such as strong coloration (in this invention, "coloration" means spontaneous coloration or discoloration and does not mean intentional coloration by addition of colorants), an insufficient mechanical strength due to the structure of a lot of crosslinking and branching and particularly thermal instability which tends to cause stronger coloration and gelation thus increasing viscosity upon melt processing.
There have been also developed processes for preparing high molecular weight linear polyarylene thioethers by a process of polymerization wherein a large amount of an organic carboxylate such as lithium acetate (Specification of U.S. Pat. No. 4,038,259) is added or a process of polymerization wherein a large amount of water is added and a simultaneously polymerization temperature is increased during polymerization (the two-step polymerization with supplementation of water, Japanese Patent Laid-Open Publication No. 14228/86). The polyarylene thioethers obtained by these processes generally have sufficiently high melt viscosity and can be subjected to melt processings such as extrusion without curing, compression molding, stretching, heat setting and the like, so that it has been expected to obtain formed products which have excellent mechanical properties and low coloration.
However, the former process has a problem that cost for production is high, and the latter process, which is excellent in economy, has also a problem that coloration tends to occur like in the case of the former process in the conventional melt processing wherein polymer powder is subjected to melting in a cylinder of an extruder, particularly a ferrous extruder.